Mechanism Insights into the Iridium(III)- and B(C6F5)3-Catalyzed Reduction of CO2 to the Formaldehyde Level with Tertiary Silanes

The catalytic system [Ir(CF3CO2)(κ2-NSiMe)2] [1; NSiMe = (4-methylpyridin-2-yloxy)dimethylsilyl]/B(C6F5)3 promotes the selective reduction of CO2 with tertiary silanes to the corresponding bis(silyl)acetal. Stoichiometric and catalytic studies evidenced that species [Ir(CF3COO-B(C6F5)3)(κ2-NSiMe)2] (3), [Ir(κ2-NSiMe)2][HB(C6F5)3] (4), and [Ir(HCOO-B(C6F5)3)(κ2-NSiMe)2] (5) are intermediates of the catalytic process. The structure of 3 has been determined by X-ray diffraction methods. Theoretical calculations show that the rate-limiting step for the 1/B(C6F5)3-catalyzed hydrosilylation of CO2 to bis(silyl)acetal is a boron-promoted Si–H bond cleavage via an iridium silylacetal borane adduct.

T he potential of CO 2 as a renewable and cheap C1 carbon source has received increasing attention over recent years. 1 The major difficulties to achieve this goal are the kinetic and thermodynamic stability of CO 2 , which hampers most of its chemical transformations.In this regard, catalysis has proven to be an essential tool for transforming CO 2 into valueadded chemicals.Although great advances have been made in the field of the catalytic transformation of CO 2 , there are still many challenges to overcome for its utilization as a raw material on an industrial scale. 2,3ormic acid, formaldehyde, methanol, and methane are C1 chemicals that can be obtained from the reduction of CO 2 .In this work, we focus on formaldehyde, which is obtained industrially by the partial oxidation of methanol and has an annual demand of 30 million tons. 4 The catalytic hydrogenation of CO 2 to formaldehyde has been scarcely reported. 5owever, several examples of the catalytic reduction of CO 2 to the formaldehyde level with hydrosilanes 6−14 or hydroboranes 15 have been reported.Catalytic systems based on Zr, 6 Re, 7 Ru, 8 Co, 9 Ni, 10 Pd, 11 Pt, 11 Sc, 12 Mg, 13 and Zn 13 complexes and germylene-B(C 6 F 5 ) 3 adducts 14 have proven to be effective for the selective reduction of CO 2 with hydrosilanes to the corresponding bis(silyl)acetal.It is noteworthy that all of these catalytic systems require the use of a Lewis acid, such as B(C 6 F 5 ) 3 , to selectively achieve the formation of the corresponding bis(silyl)acetal. 16The selectivity of these processes depends on the metal/B(C 6 F 5 ) 3 ratio.−14 Although the effectivity of B(C 6 F 5 ) 3 as a hydrosilylation catalyst is well-known, 17 B(C 6 F 5 ) 3 alone cannot catalyze the hydrosilylation of CO 2 .6a, 18 It has recently been proven that bis(silyl)acetal, H 2 C-(OSiPh 3 ) 2 , provides a means to incorporate CH n (n = 1 or 2) moieties into organic molecules. 19Therefore, developing catalytic systems effective for the reduction of CO 2 to the bis(silyl)acetal level using hydrosilanes is of great interest.Understanding the mechanisms that operate in different transition-metal-catalyzed processes to reduce CO 2 with hydrosiloxanes is one of our aims. 20We have recently reported that species [Ir(CF 3 CO 2 )(κ 2 -NSi Me ) 2 ] [1; NSi Me = (4methylpyridin-2-yloxy)dimethylsilyl] catalyzes the selective reduction of CO 2 with HSiMe(OSiMe 3 ) 2 to the corresponding methoxysilane, CH 3 OSiMe(OSiMe 3 ) 2 , or silylformate, HCO 2 SiMe(OSiMe 3 ) 2 , under mild reaction conditions.The selectivity of this catalytic system can be easily tuned by controlling the pressure of CO 2 . 21It is noteworthy that the two active positions of the catalytic systems based on 1 are trans located to two silyl groups; in addition, the Ir−Si bond in such species is stronger than would be expected for a traditional Ir− silyl bond. 22Hence, the positions trans to the Ir−Si bonds in Ir(κ 2 -NSi Me ) 2 complexes are highly activated.
We now report that using 1 as a catalyst precursor in the presence of catalytic amounts of B(C 6 F 5 ) 3 allow achievement of the selective formation of bis(silyl)acetals by the reaction of CO 2 with hydrosilanes (Scheme 1). 1 H NMR studies of the 1/B(C 6 F 5 ) 3 (1:1 ratio; 1.0 mol %)-catalyzed reaction of CO 2 (1 bar) with HSiMe(OSiMe 3 ) 2 (HMTS) in C 6 D 6 at 323 K show the slow and selective formation of H 2 C{OSiMe(OSiMe 3 ) 2 } 2 (2a; Table 1, entry 1).To explore the scope of this catalytic process, we performed the reaction of CO 2 with different silicon hydrides (HSiMe 2 Ph, HSiMePh 2 , HSiEt 3 , and HSiMe(OSiMe 3 ) 2 ) in the presence of 1/B(C 6 F 5 ) 3 (1:1) in C 6 D 6 .The best reaction conditions were found to be CO 2 (1 bar) and 323 K.The reactions are highly selective to the formation of the corresponding bis(silyl)acetal (Table 1, entries 1, 2, 4, and 5).The nature of silane influences the reaction performance.The best reaction rates were obtained using HSiMe 2 Ph and HSiMePh 2 (Table 1).The reactions with HMTS and HSiEt 3 were slower, which can be attributable to the higher hindrance of the Si−H bond in such compounds.
1 H NMR studies of the 1/B(C 6 F 5 ) 3 (1:1; 1.0 mol %)-catalyzed reaction of CO 2 with HSiMe 2 Ph in C 6 D 6 at 323 K demonstrate the influence of CO 2 pressure on the reaction performance; at 3 bar, the reactions are faster but less selective than those at at 1 bar (Table 1, entries 2 and 6).The stoichiometry of borane is a key factor in the selectivity of these catalytic processes.Within the range of 1−3 bar of CO 2 , if the load of B(C 6 F 5 ) 3 is increased from 1.0 to 2.0 mol %, the reactions are selective toward the formation of methane 23 and O(SiMe 2 Ph) 2 , albeit at a lower rate (Table 1, entry 7).While reducing the amount of B(C 6 F 5 ) 3 to 0.5 mol % does not alter the activity, the selectivity is affected, resulting in the formation of silylformate (82%) and bis(silyl)acetal (18%) as secondary products (Table 1, entry 8).In the absence of additives, the catalyst precursor 1 promotes the reduction of CO 2 (1 bar) with HSiMe 2 Ph to give silylformate (90%) as major reaction product (Table 1, entry 9). 1 H NMR studies of the 1-catalyzed (1.0 mol %) reaction of CO 2 (1 bar) with HSiMe 2 Ph in the presence of BPh 3 (1.0 mol %), instead of B(C 6 F 5 ) 3 , show a slower and less selective reaction.After 24 h, a 73% conversion of hydrosilane is reached to give a mixture of the corresponding silylformate (81%), bis(silyl)acetal (12%), and methoxysilane (7%) (Table 1, entry 3).Therefore, BPh 3 plays a role in the activity and selectivity of the process, although to a lesser degree than B(C 6 F 5 ) 3 , which can be correlated to its lower Lewis acidic character. 24 1  NMR studies of the reaction of 1 with B(C 6 F 5 ) 3 evidenced the quantitative formation of [Ir(CF 3 COO-B- Contrarily, no reaction is observed between 1 and BPh 3 under the same conditions.The molecular structure of 3 has been confirmed by X-ray diffraction studies (Figure S38).The  The 11 B{ 1 H} NMR spectra of 3 show a singlet at δ = −1.7 ppm (Figure S18), in agreement with what is expected for the O−B(C 6 F 5 ) 3 fragment 25 (Scheme 2).The absolute value of the difference between δ para and δ meta of the fluorine atoms Δ(δ m,p ) in the 19 F NMR spectra of 3 is 6.3 ppm (Figure S21), which agrees with the presence of a tetracoordinated borate anion. 26,27he addition of 1 equiv of HSiMe(OSiMe The addition of an excess of HSiMe(OSiMe 3 ) 2 to C 6 D 6 solutions of 3 produces 4 and CF 3 CH{OSiMe(OSiMe 3 ) 2 } 2 .Note that the overreduced product CF 3 CH 2 OSiMe(OSiMe 3 ) 2 is not obtained, which is reminiscent of the 1/B(C 6 F 5 ) 3 system selectivity toward the bis(silyl)acetal species.This evidences the effective entrapment of B(C 6 F 5 ) 3 in the form of a hydridoborate ion pair because the free borane might promote activation of the Si−H bond toward reduction of the bis(silyl)acetal derivatives, as well as the direct participation of 4 in the catalytic reaction, because 4 not only promotes hydrosilylation of the TFA ligand or CO 2 but also catalyzes reduction of the R′COOSiR 3 species (R′ = H, CF 3 ).
The 1 H NMR spectra of 4 in C 6 D 6 show no changes when pressurized with CO 2 (3 bar) at RT.However, after the reaction mixture is heated at 323 K, the formation of complex [Ir(HCOOB(C 6 F 5 ) 3 )(κ 2 -NSi Me ) 2 ] ( 5) is observed.The presence of a IrOC(H)OB(C 6 F 5 ) 3 moiety in 5 has been demonstrated by means of 1 H, 13 C, 11 B, and 19 F NMR spectroscopies (Figures S32−S36).The addition of 2 equiv of HSiMe(OSiMe 3 ) 2 to a solution of 5, in the absence of CO 2 , produces the formation of 2a and the regeneration of 4 within 1 h at RT (Scheme 2).Exposure of 5 to 13 CO 2 (2.7 bar) at 353 K for 48 h did not result in the partial substitution of [Ir]OC(H)OB(C 6 F 5 ) 3 to the 13 C-enriched [Ir]O 13 C(H)OB-(C 6 F 5 ) 3 , which suggests that, different from that reported for analogous MOC(H)OB(C 6 F 5 ) 3 (M = Re, 8 Ni, 10 Pd, 11 Pt 11 ) species, the CO 2 insertion step to give 5 is irreversible under the catalytic conditions.
Density functional theory (DFT) studies at the M06L-(SMD)/def2-TZVP//B3LYP-D3(BJ)/def2-SVP level have been performed to study in detail the reaction mechanism of CO 2 hydrosilylation catalyzed by 3 (see the SI).HSiMe 3 has been selected as a model system for the silanes.The Gibbs free energy energetic profile for the catalyst activation process, from 3 (A) to 4 (D) (Figure S39), is exoergic by 10.8 kcal mol −1 .Si−H bond activation occurs via boron-promoted Si−H cleavage TSBC (9.0 kcal mol −1 ), which corresponds to a linear S N 2 nucleophilic attack of the terminal oxygen of the trifluoroacetate ligand to the silicon atom in which the leaving hydride is transferred to the boron moiety.A similar type of activation mechanism has been proposed for Lewis acid PBP− Ni hydrosilylation of CO 2 based on DFT calculations.10b An alternative mechanism for the Si−H activation step based on a nickel-promoted Si−H cleavage has been proposed.10c In our case, the iridium-promoted Si−H cleavage is energetically disfavored (see Figures S40 and S41 for a comparison of both pathways).Intermediate D can be described as a hydroborate moiety and a cationic metallic complex rather than a metal hydride interacting with the Lewis acid.Inspection of the natural bond orbitals reveals a σ(B−H) bonding orbital with an electron population of 1.76 electrons (Figure S42).
The coordination of CO 2 to D leads to the beginning of the catalytic cycle.The Gibbs free energy profile for this process is reported in Figure 1.The first step corresponds to hydride transfer from HB(C 6 F 5 ) 3 to CO 2 via TSEF at an energy barrier of 22.2 kcal mol −1 from intermediate D. The obtained intermediate F is thermodynamically favored (−13.1 kcal mol −1 ) and corresponds to complex 5 experimentally detected by NMR.Following that, the addition of silane leads to σ 1 -H-(HSiMe 3 ) coordination to F, yielding G.Then, activation of the Si−H bond takes place via TSGH, like the previously reported TSBC, consisting of the linear S N 2 nucleophilic attack of the terminal oxygen atom of the formate to the silicon atom and transfer of the leaving hydride to the boron atom of the Lewis acid.The activation barrier of TSGH is 15.9 kcal mol −1 , leading to intermediate H.The subsequent hydride transfer from the hydroborate to the carbon atom of the silylformate coordinated to the metal takes place through TSHI, yielding intermediate I. Upon reaction with another molecule of HSiMe 3 , the silylformate develops into the final bis(silyl)acetal product via TSIJ, with the activation energy for this step being 23.2 kcal mol −1 .This activation barrier for the boronpromoted Si−H bond cleavage is higher than those of the previously related processes, TSBC (9.0 kcal mol −1 ) and TSGH (15.9 kcal mol −1 ).It should be noted that, for TSIJ, the nucleophilic attack to the silane is performed by an alkoxy group, 10,17c,28 in contrast with previous steps, where the nucleophilic attack was performed by trifluoroacetate and formate groups.
The catalytic process is strongly exergonic (−30.9 kcal mol −1 ), and the rate-limiting step is boron-promoted Si−H cleavage by the iridium silylacetal borane adduct I (23.2 kcal mol −1 ) characterized by TSIJ.This activation barrier agrees with the experimental finding that the reaction proceeds slowly at RT. Indeed, the reaction of 4 with CO 2 (3 bar) to give 5 requires heating at 323 K.It should be noted that the intermediates proposed in the DFT-calculated catalytic cycle match the experimentally detected species (4 and 5; Scheme 2).
In conclusion, this is the first example of an iridium-based catalytic system effective for the selective reduction of CO 2 to the formaldehyde level with hydrosilanes.The selectivity of this catalytic system to the formation of bis(silyl)acetals is determined by the interaction between the active species and the Lewis acid B(C 5 F 6 ) 3 .In fact, any factor that affects that interaction influences the selectivity of the process.Thus, using a borane with a lower Lewis acidity such as BPh 3 , high temperature, or CO 2 pressure higher than 1.0 bar inhibit the selectivity toward the bis(silyl)acetal.DFT calculations support a boron-promoted Si−H cleavage mechanism, with the ratelimiting step being boron-promoted Si−H cleavage by the iridium silylacetal borane adduct I.

Figure 1 .
Figure 1.DFT-calculated Gibbs free energy profile for the catalytic formation of bis(silyl)acetal from E (kcal mol −1 ) relative to A.